1. Field of the Invention
This invention relates generally to conveyor belts, and is concerned in particular with the provision of a conveyor belt comprised essentially of polytetrafluoroethylene (“PTFE”).
2. Description of the Prior Art
PTFE resins possess many beneficial properties, such as excellent chemical resistance, high temperature capabilities, and superb release characteristics. Over time, it has proven very useful to access these properties in conveyor belting service for many industries, including for example food preparation, packaging, and chemical processing.
Conventionally, PTFE used in belting conveyor service have been coupled with some form of reinforcement for support. Typically, the reinforcement consists of a woven textile. The PTFE polymer is typically applied to the woven textile either by a coating process, or via lamination. By far, the main material used for reinforcement in PTFE conveyor service is woven fiberglass. Other materials are also used, such as Kevlar and Nomex products, but they are more costly and tend to be less effective due to temperature and/or chemical limitations.
Woven fiberglass, as a reinforcement for PTFE, offers many advantages and, unfortunately, a number of disadvantages. On the plus side, the fiberglass material is economical and possesses excellent high temperature properties. It is extremely strong on a strength/weight basis. Also, for many applications it provides good resistance to chemicals.
On the negative side, the woven fiberglass support is very susceptible to flex fatigue failure. If it encounters a point stress or load in belting service, it often will tear or crease, causing irreversible damage to the belting material. Because of the fatiguing properties, it is typically necessary to give consideration to belt rollers with larger diameters to minimize belt flexing. Very importantly, because fiberglass is a high modulus material, it does not readily accommodate elongation in belting service. This can make the belting product extremely difficult to track in service. In addition, fiberglass can be readily attacked by most of the chemicals that succeed in penetrating through the PTFE coating/film residing on the fiberglass surface.
Additionally, because both PTFE resins and fiberglass have very high temperature capabilities, many applications rely on transferring heat through the belting product. In these applications, thin woven fiberglass materials are required so that the heat can be effectively conducted. To achieve the required minimum thickness and low weight, the PTFE/fiberglass belting materials are produced on fine fiberglass fabrics that are particularly susceptible to damage in belting use. Some of the most severe belting applications involve the use of PTFE/fiberglass materials in cooking foods such as for example bacon, chicken, and hamburgers, at elevated temperatures on the order of 500° F.
Finally, the PTFE/fiberglass interface in belting products is a weak structural link. Because PTFE resins are difficult to bond to, high adhesions can be difficult to achieve at the PTFE/fiberglass interface. Also, the continual flexing of the belting material in service regularly breaks down the often tenuous adhesion between the fiberglass and the PTFE polymer. These factors usually lead to blistering or delamination of the PTFE component in the product. Blistering is a common cause for failure in PTFE/fiberglass belts cooking greasy foods at elevated temperatures.
In spite of the problems and difficulties noted above, and as demonstrated for example by the disclosures in U.S. Pat. Nos. 5,077,072; 5,388,503; 5,588,354; 5,951,895; 6,919,122; and WO 96/16583, those skilled in the art have continued to rely on reinforced PTFE materials for belting applications. This has been due in large part to the mistaken assumption that unreinforced PTFE has insufficient resistance to deformation, and that it lacks the strength properties to survive in belting applications, in particular those having hostile environments that involve exposure to corrosive chemicals, high temperatures and/or challenging mechanical loads.
The present invention stems from the surprising discovery that contrary to conventional thinking, an unreinforced PTFE film laminate, when properly constituted and conditioned, can serve as a reliable conveyor belting product under the most demanding conditions.